Abstract
The type III secretion system (T3SS) is a complex nanomachine of many pathogenic Gram-negative bacteria. It forms a proteinaceous channel that is inserted into the host eukaryotic cell membrane for injection of bacterial proteins that manipulate host cell signaling. However, few studies have focused on the effector-independent functions of the T3SS. Using a murine model of acute lung infection with Pseudomonas aeruginosa, an important human opportunistic pathogen, we compared the pathogenicity of mutant bacteria that lack all of the known effector toxins ( ΔSTY), with mutant bacteria that also lack the major translocator protein PopB (ΔSTY/ΔPopB) and so cannot form a functional T3SS channel in the host cell membrane. Mortality was higher among mice challenged with ΔSTY compared to mice challenged with ΔSTY/ΔPopB mutant bacteria. In addition, mice infected with ΔSTY showed decreased bacterial clearance from the lungs compared to those infected with ΔSTY/ΔPopB. Infection was in both cases associated with substantial killing of lung infiltrating macrophages. However, macrophages from ΔSTY-infected mice died by pro-inflammatory necrosis characterized by membrane permeabilization and caspase-1 mediated IL-1β production, whereas macrophages from ΔSTY/ΔPopB infected mice died by apoptosis, which is characterized by annexin V positive staining of the cell membrane and caspase-3 activation. This was confirmed in macrophages infected in vitro. These results demonstrate a T3SS effector toxin independent role for the T3SS, in particular the T3SS translocator protein PopB, in the pathogenicity of P. aeruginosa during acute lung infection.
Highlights
Pseudomonas aeruginosa is a Gram-negative bacterium found ubiquitously in soil and water habitats
DSTY bacteria do not produce the effector toxins ExoS, ExoT and ExoY expressed by wild type (WT) bacteria, and so these T3SS effector proteins probably do not play a role in death from acute lung infection with P. aeruginosa
Much effort has been put in the functional and structural characterization of the T3SS of P. aeruginosa and other Gramnegative bacteria, and its interaction with the host cell. These studies have focused mainly on the role of the T3SS effector proteins (ExoS, ExoT, ExoY, ExoU in the case of P. aeruginosa) in virulence. While this manuscript was under revision, T3SS rod and needle proteins were shown to trigger the activation of the NAIP/NLRC4 inflammasome when expressed in mammalian cells [24,25,26], illustrating functions of the T3SS beyond the injection of T3SS effector proteins
Summary
Pseudomonas aeruginosa is a Gram-negative bacterium found ubiquitously in soil and water habitats. Most clinical isolates of P. aeruginosa secrete virulence determinants and possess a specialized proteinaceous apparatus associated with the cell wall which is used to translocate toxins into eukaryotic cells This is known as the type III secretion system (T3SS). The translocation apparatus is a membrane pore that accepts the effector proteins secreted by the needle complex and delivers them across the host cell plasma membrane. PopB, PopD and PcrV are secreted via the T3SS and are absolutely required for pore formation and translocation of effectors across the host cell plasma membrane [7,8]. Similar to ExoS, ExoT and ExoY, ExoU requires either a eukaryote-specific cofactor for its activity and ubiquitinated proteins, as well as ubiquitin itself, have been suggested as being potential activators of the toxin [20]. Both genotypes (ExoS/ExoT and ExoU/ExoT) are associated with acute infections in humans, though ExoU-producing strains are under-represented in persistently infected cystic fibrosis patients [21]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
